On high-exertion outings, it's easy to overdress and overheat–then get chilled in a sweaty baselayer. This shirt's combination of warm merino wool in the torso, paired with ventilated merino on the sides, upper back, and under the arms (from armpit to wrist), helps you go hard without getting soaked in those sweat-prone areas. And you can fine-tune temperature control with a deep chest zip and to-the-chin collar that seals closely around your neck.
The lightweight merino is best for shoulder-season temps or hot-running bodies; Icebreaker makes several heavier versions for when you need more warmth. All of them work great on long winter trips, when merino's natural odor control will keep shirts stink-free for a week. The body-hugging fit and hip-level key pocket also make this a great winter trail-running shirt. The women's version is called the Dash. $90; 7 oz.; men's S-XXL, women's S-XL; icebreaker.com